In joint arthroplasty procedures, positioning of prosthetic implant or components is one mechanism used in restoring joint functionality. For instance, in knee arthroplasty procedures, positioning of femoral, tibial and/or patellar components is generally necessary in restoring natural mechanical alignment between the distal femur and proximal tibia, for example. Natural mechanical alignment is generally defined as the angle between the femoral and tibial mechanical axes.
Patients typically have some deviation between the femoral and the tibial mechanical axes, resulting in a natural varus or valgus orientation of the knee joint. This deviation generally increases due to natural degeneration and wear or may increase due to injury. Therefore, it is often a goal of knee arthroplasty procedures, for example, to restore the patient's natural varus/valgus orientation.
Two common approaches to orthopaedic implant placement in knee arthroplasty procedures are classical mechanical alignment and anatomic alignment. In classical mechanical alignment, the tibia is generally resected perpendicular to the patient's tibial mechanical axis. Aligning a tibial implant perpendicular to the tibial mechanical axis will generally load each condyle equally and will align joint forces in a compressive orientation down the axis of the tibia. This loading is beneficial due to the orthotropic nature of bone that causes it to be strongest in axial loading such that joint forces are spread out equally on both the medial and lateral condyles. The patient's natural deviation from mechanical alignment is typically compensated by making an ideally equal and opposite cut on the femur in flexion and extension. This may lead, however, to removing more bone than is desirable. With an anatomic approach, tibial cuts are oriented at a slope towards the patient's natural varus/valgus alignment. This approach may better recreate the patient's natural kinematics and conserve bone by avoiding compensatory cuts. However, one problem that has been recognized is that such an approach may also lead to tibial baseplate loading that is off axial, providing a mechanical disadvantage at the bone/device interface by introducing shear loads, for example.
In one analysis of 544 patient tibiae, it was shown that tibiae have on average a 2.76 degree of natural tibial varus and thus have a lateral condyle that is located superiorly to the medial condyle relative to the tibial mechanical axis. Literature sources have shown that healthy patients tend to have a similar degree of joint and tibial varus. Publications have also shown more extreme joint deviations from mechanical in osteoporotic patients, which on average tend to be in tibial varus. Noting these findings, it would indicate that mechanical surgical preparation on the tibia would tend to resect relatively more bone on the lateral tibia, and require an equivalent over resection on the medial femur. It would also indicate that anatomic alignment would, on average, place implants in 2-3 degrees tibial varus resulting in off axis loading.